This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Background: NASA scientists are working to find ways to keep astronauts healthier and safer when they spend a long time in space. Head down bed rest is a good way to pretend a person is traveling in space without gravity. Specific Aims and Procedures: Subjects in this study will live at the special research unit at the hospital in Galveston for 114-days. They will stay in bed for 90 of those days. Along with bed rest, these studies will have mental and physical tests called the Integrated Testing Regimen (ITR) and other studies of 5 scientists. These tests measure changes in the body and mind before, during and after bed rest. Right now, many of these tests are used on astronauts before and after space flight to watch the changes from long trips into space. Experimental Design (summary): In order to learn as much as possible during the study the scientists at NASA will do the set of tests that they have developed over many years for evaluating astronauts. In addition, there are five other sets of tests that will help all of the scientists learn new and better ways to help keep the astronauts healthy Significance (summary): NASA scientists hope to get information that will help keep astronauts healthy on future space flights.